The present invention relates to a method for detecting a slightly irregular surface state present on a flat surface of a sample such as a silicon wafer or a glass substrate to be used in a liquid crystal display device. More particularly, it relates to a method for detecting and examining a slightly irregular surface state which enables facilitated observation, analysis, examination and evaluation on a slightly irregular surface state, such as on the three dimensional shape of each slightly irregular surface state, by detecting and locating the slightly irregular surface state using a particle detector having its own coordinate system and linking the coordinates of the slightly irregular surface state to the coordinate system of another analyzer such as a scanning probe microscope thereby specifying the location of the slightly irregular surface state in terms of the coordinate system of the analyzer. The present invention also relates to a scanning probe microscope imparted with functions for the method and to a method for fabricating a semiconductor device or a liquid crystal display device using the foregoing method and microscope.
In the present invention, the words "slightly irregular surface state" includes, for example, a state where fine particles are adhered on a surface, a condition where fine or micro projections and fine or micro pits are present, an abnormal condition caused by a crystal defect or the like. Further, the term "analyzer such as a scanning probe microscope" is meant to include an observing apparatus, an analyzing apparatus, an examining apparatus and an evaluating apparatus, each having an atomic force microscope (AFM), a scanning tunnel microscope (STM), a magnetic force microscope (MFM) or the like.
In the manufacture of VLSIs, typically a 4 Mbit- or 16 Mbit-DRAM, the production yield is known to almost depend upon failures attributable to slightly irregular surface states, such as fine particles adhering to a wafer. This is because with increasingly miniaturizing pattern width slightly irregular surface states which have not been considered critical come to play the role of contaminants. In general, the size of a slightly irregular surface state herein in question is considered as small as the minimum interconnection line width of a VLSI be manufactured reduced by a factor of several units. From this, in the manufacture of a 16 Mbit-DRAM of which the minimun interconnection line width is 0.5 .mu.m, a slightly irregular surface state of the order of 0.1 .mu.m diameter is of concern. Such a slightly irregular surface state acts as a contaminant and hence is responsible for breaks, shortcircuiting of a circuit pattern or the like. This leads to products prone to failure, hence, products of degraded reliability and quality. For that reason, in improving the production yield it is a critical point that the realities of slightly irregular surface states, for example in what conditions they adhere to a sample, be determined and controlled by quantitative measurement and analysis with high precision.
As the means to accomplishing this end, a particle detector has been conventionally used which is capable of detecting a slightly irregular surface state present on a surface of a flat sample such as a silicon wafer. Examples of such conventional particle detectors include IS-2000 and LS-6000, products of Hitachi Electronics Engineering Co., Ltd., SURFSCAN 6200, a product of Tencor Co., USA, and WIS-9000, a product of Estek Co., USA. The measuring principle underlying these particle detectors and the system configuration for realizing these detectors are described in detail in, for exmaple, "Technology for analyzing and evaluating high performance semiconductor processes" edited by Semiconductor substrate technology studies, published by REALIZE Co., Ltd., pp. 111-129.
FIG. 11 shows the results of a measurement on slightly irregular surface states (each larger than 0.1 .mu.m in size) present on an existing 6 in. silicon wafer. In FIG. 11 the circle represents the contour of the wafer and the points in the circle correspond to the locations of the slightly irregular surface states.
As seen from FIG. 11, however, the conventional particle detector merely provides such information as the locations and distribution of particle sizes of slightly irregular surface states present on a surface of the sample such as a silicon wafer and cannot identify the realities of the slightly irregular surface states, for example, what forms each slightly irregular surface state.
Also, there has been conventionally used a scanning probe microscope having a high resolution power, such as an atomic force microscope or a scanning tunnel microscope, to observe the three-dimensional microscopic shapes of slightly irregular surface states on a surface of a flat sample such as a silicon wafer.
The atomic force microscope is a device to observe the three-dimensional shape of a surface of a sample by bringing near the surface of the sample a projecting pyramid probe needle such as of Si.sub.3 N.sub.4 mounted on the tip of a cantilever, scanning the sample in the x-y plane while adjusting the level z of the sample so as to keep the Van Del Waals force exerted between the sample and the probe needle constant (typically about 10.sup.-9 N), and monitoring z-axis control signals fed in the scanning operation.
FIGS. 12(a) and 12(b) illustrate the basic configuration of a conventional atomic force microscope (for example, NanoScope AFM produced by Digital Instrument Co.) for use in observing a surface of a sample such as a silicon wafer. In FIG. 12 a probe needle 1 for scanning a surface of a sample 2 is a pyramid-like projection of Si.sub.3 N.sub.4 mounted on the tip of a cantilever 3. When the probe needle 1 is brought near the sample 2, the cantilever 3 is bent because of the repulsive force produced by mutual contact of atoms. The degree of bending of the cantilever 3 is proportional to the atomic force exerted between the probe needle 1 and the sample 2. The bending of the cantilever 3 is detected by utilizing the variation in the reflecting direction of bending detection laser beam 4 emitted from a laser light source 5 comprising a light-emitting device such as a semiconductor laser and illuminated onto the reflecting surface of the cantilever 3. This is referred to as "optical lever method". Light reflected by the cantilever 3 is detected by a light-receiving device 6 such as a photodiode. While controlling the level of the sample so as to keep the bending of the cantilever 3 constant, the sample is scanned in x-y plane by operating an xyz stirring actuator 7 to measure the three-dimensional shape of the surface of the sample. Control signals applied to the actuator 7 for the movement along each axis in this scanning operation are input to a microcomputer, graphically processed and displayed as the results of the three-dimensional measurement on the sample surface.
The scanning tunnel microscope (STM) is a device for observing the three-dimensional shape of a surface of a sample by bringing a metallic probe needle very near (about 1 nm) the sample, scanning the sample in x-y plane with the metallic needle while controlling the level z of the sample so as to keep the tunnel current flowing between the sample and the needle constant, and monitoring control signals applied for the movement of the sample along z axis in this scanning operation.
FIG. 13 illustrates the basic configuration of a conventional scanning tunnel microscope (for example, NanoScope STM produced by Digital Instrument Co.) for use in observing a surface of a sample such as a silicon wafer. In FIG. 13 a metallic probe needle 21 for scanning a surface of a sample 2 comprises, for example, tungsten filament and is shaped acute at the tip thereof by electropolishing. A bias voltage is applied across the probe needle 21 and the sample 2 from a DC power source 23.
In the three-dimensional measurement of the sample surface, the metallic probe needle 21 applied with bias voltage is brought near the surface of the sample by means of an xyz stirring actuator 27 so as to allow a tunnel current of a predetermined magnitude to flow between the two, the tunnel current being measured and controlled by an ammeter 24. In turn, while controlling the level z of the metallic probe needle 21 so as to keep the magnitude of the tunnel current constant, the probe needle 21 is made to scan the sample in x-y plane by means of the xyz stirring actuator 27. Control signals applied to the actuator 27 for the movement along each axis in this scanning operation are input to a microcomputer, graphically processed and displayed as the results of the three-dimensional measurement on the sample surface.
Incidentally, the conventional atomic force microscope and scanning tunnel microscope are described in detail in, for example, "THE TRC NEWS", vol. 38 (January 1992), pp. 33 to 39, published by Toray Research Center Co., Ltd., and "STM book for easy reading" published by Hitachi Ltd., 1991.
Now, there is a desire to identify slightly irregular surface states in actual conditions by directly observing an individual slightly irregular surface state or analyzing the same to determine the composition thereof with the use of an appropriate analyzer such as a scanning probe microscope. With regard to the conventional analyzers, however, the scanning probe microscope for instance is adapted to bring its probe needle into contact with a surface of a sample at any location and scan the surface, and hence is designed to observe only the condition of the sample surface where the probe needle is able to scan. Therefore, it is very difficult for the scanning probe microscope to position the probe needle thereof at a location where a slightly irregular surface state is present. Accordingly, the scanning probe microscope is unable to satisfactorily respond to the demand to find out and examine very few slightly irregular surface states of submicron order or smaller, such as crystal defects, which are present on a surface of a sample. For instance, when a wafer size of which is 6 in. is compared to the area of the Sado island (area of which is 857 km.sup.2) of Japan, a slightly irregular surface state of 0.3 .mu.m size is equivalent in size to a golf ball. It is very hard to detect such a slightly irregular surface state, specify the location thereof and position the probe needle at such location. This makes it nearly impossible to find out any slightly irregular surface state of submicron order or sub-submicron order such as a crystal defect and observe it three-dimensionally. Further, even if the particle detector is previously used to locate a slightly irregular surface state, the location thereof is defined in terms of the coordinate system of the particle detector and, hence, it is difficult to define the location of the slightly irregular surface state in terms of the coordinate system of the analyzer. Still further, since the location of individual slightly irregular surface states on a wafer is defined by a pixel area (typically of 20 .mu.m.times.200 .mu.m) which depends upon the size of laser beam focused on the wafer, the location is defined with an inherent error equivalent to the area of the pixel used. Where the sample having been subjected to slightly irregular surface state detection by the particle detector is to be set in an analyzer, such as scanning probe microscope, other than the particle detector, an additional error due to shifting of the coordinates is inevitable because of the setting again. For that reason, to identify the realities of a slightly irregular surface state certain measures are required to be taken to completely link the coordinate system of the particle detector to that of the analyzer such as the scanning probe microscope and eliminate the error attributable to a pixel area, or to register the coordinates of the slightly irregular surface state specified by the particle detector into the coordinate system of the analyzer so as to eliminate the error by an approach such as to newly detect the location of a slightly irregular surface state again specified by the particle detector. Note that the focused beam area is about 20 .mu.m.times.200 .mu.m, which is described in the foregoing literature "Technology for analyzing and evaluating high performance semiconductor processes".
Various particle detectors and scanning probe microscopes were examined for coordinate systems of their x-y stages. As a result, almost all of them were found to employ x-y coordinate system. The coordinate axes and the origin with respect to a wafer as a sample to be measured are determined by the following methods: (1) to assume the flat axis of the orientation flat of the wafer be the x axis (or y axis), a normal line to the x axis in the wafer plane be the y axis ( or x axis ), the intersecting point of the circumference of the wafer and the y axis be a point (0,y), and the intersecting point of the cricumference and the x axis be a point (x,0); and (2) to assume the flat axis of the orientation flat of the wafer be the x axis (or y axis), a normal line to the x axis in the wafer plane be the y axis, and the center of the wafer determined from the equation of circle using three points obtained on the circumference (except the orientation flat) of the wafer be the origin ( 0,0).
With the above methods, however, these coordinate axes and the origin or center necessarily deviate from one wafer to another or for each setting because of the difference in the circumferential surface precision or in the precise size between wafers or because of the difference between respective positions of wafers set on the stage or of subtle warpage involved in each wafer. As a result, deviation of the coordinate axes and origin with respect to individual wafer is inevitable between the devices employing these methods (e.g., between the particle detector and the analyzer such as scanning probe microscope). Various devices were examined for the amount of a deviation caused by the abovementioned reasons by the use of several wafers each having a lattice pattern. The examination revealed the fact that even between devices of high precision (particle detector: IS-2000 produced by Hitachi Electronics Engineering Co., Ltd., measure SEM: S-7000 produced by Hitachi Ltd.) the origin or center of x-y coordinates and any point defined in terms of the x-y coordinates deviated about (.+-.100 .mu.m,.+-.100 .mu.m). For that reason, when a slightly irregular surface state present on a wafer at any location which has been detected by the particle detector is to be observed, analyzed and evaluated using the analyzer such as the scanning probe microscope, one must observe the region (200 .mu.m.times.200 .mu.m=40000 .mu.m.sup.2) covering the regional range of (.+-.100.mu.m,.+-.100 .mu.m) or more from the center, or the location where the slightly irregular surface state is considered to be present by the use of the scanning probe microscope to confirm the location of the slightly irregular surface state, and then observe or analyze the slightly irregular surface state by enlarging the slightly irregular surface state or a like process to serve the purpose. For that reason, it takes considerably time for observing or analyzing the slightly irregular surface state.
Attempt is to be made to appreciate the dimensions of the aforesaid region relative to the slightly irregular surface state. Assuming that the region of 40000 .mu.m.sup.2 (200 .mu.m.times.200 .mu.m) is observed using a CCD camera having one million pixels which is considered to have a relatively high resolution power, the detection range (area) covered by one pixel of the CCD camera is calculated to make consideration on the size of a smallest detectable slightly irregular surface state. Under the above conditions the detection range covered by one pixel is found to be 0.04 .mu.m.sup.2 from the calculation: 40000 .mu.m.sup.2 .div.1000000=0.2 .mu.m.times.0.2 .mu.m. Since a slightly irregular surface state of the size not larger than one pixel is difficult to identify, the limit in the detection of a slightly irregular surface state is 0.04 .mu.m.sup.2 (0.2.mu.m.times.0.2 .mu.m). Stated otherwise, it is difficult to directly detect a slightly irregular surface state having a projected area of not larger than 0.04 .mu.m.sup.2 (equivalent to about 0.2 .mu.m diameter) by means of the CCD camera having one million pixels and, hence, it seems nearly impossible to specify the location of such a slightly irregular surface state.
From the above, it can be understood that it is difficult to directly observe or evaluate a slightly irregular surface state of not larger than about 0.2 .mu.m diameter detected by the conventional particle detector by linking the coordinates of the slightly irregular surface state specified by the detector to the coordinate system of the analyzer such as the scanning probe microscope to specify the location of the slightly irregular surface state in terms of the coordinate system of the analyzer.
The present invention is attained to overcome the foregoing problems. It is, therefore, an object of the present invention to provide a method for easily finding out the location of a slightly irregular surface state present on a surface of a sample, a method for examining a slightly irregular surface state which is capable of selectively and three-dimensionally observing only a defective portion thus detected of the sample surface, and a scanning probe microscope therefor.
It is another object of the present invention to provide a method and apparatus for observing, analyzing and evaluating a slightly irregular surface state by again detecting the slightly irregular surface state of which location is previously detected in terms of the coordinate system of the particle detector by the use of the coordinate system of an analyzer other than the particle detector and linking or registering the location of the slightly irregular surface state specified by the particle detector to the coordinate system of the analyzer with good precision.
It is yet another object of the present invention to provide method and apparatus for observing, analyzing and evaluating a slightly irregular surface state by newly detecting a slightly irregular surface state which cannot be detected by the particle detector by the use of the coordinate system of an analyzer other than the particle detector and registering the location of the slightly irregular surface state in the coordinate system of the analyzer with good precision.
It is a further object of the present invention to improve the yield of semiconductor devices or liquid crystal display devices production and upgrade the reliability of the semiconductor devices or liquid crystal display devices by examining a slightly irregular surface state present on a wafer or a transparent insulative substrate by means of the aforesaid scanning probe microscope in a fabrication process of a semiconductor device or a liquid crystal display device.